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Antennae Galaxies closer than you think BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: May 12, 2008

New research from Hubble shows that the Antennae Galaxies, an interacting pair that are used as a standard candle against which to validate theories about galaxy evolution, are 20 million light years closer than previously thought.

An international team of scientists led by Ivo Saviane from the European Southern Observatory used the Hubble Space Telescope to observe individual stars spawned by the collision of the two interacting galaxies. The team targeted a region in the relatively quiescent southern tail in order to observe the older red giant stars that are known to reach a standard brightness. The brightnesses were then used to infer a distance of 45 million light years, instead of the 65 million light years previously believed.

The Antennae Galaxies, also known as NGC 4038 and NGC 4039, are named for the two long trailing tails of stars, gas and dust that resemble the antennae of an insect, and can be found in the constellation of Corvus. Left: Ground-based image taken by Robert Gendler showing the impressively long tails of the merging Antennae Galaxies. Right: Hubble Advanced Camera for Surveys image showing a portion of the southern tail (region bounded by the box in the left image). The main visible component consists of young blue stars, and many red background stars can also be seen. Image: NASA, ESA and Ivo Saviane, ESO/Robert Gendler.

The Antennae Galaxies are among the closest known merging galaxies and as a result are also the best-studied, with a wealth of observational data available to compare with the predictions of theoretical models. "All aspiring models for galaxy evolution must be able to account for the observed features of the Antennae Galaxies, just as respectable stellar models must be able to match the observed properties of the Sun,” says Ivo Saviane. “Accurate models require the correct merger parameters, and of these, the distance is the most essential".

Values as high as 100 million years had also been used previously, and these large distances required astronomers to invoke some quite exceptional physical characteristics to account for the features observed in the system, including a very high star-formation rate, supermassive star clusters and ultra-luminous X-ray sources. The new smaller distance makes the Antennae Galaxies less extreme in terms of the physics needed to explain the observed phenomena. Its infrared radiation is now that expected of a 'standard' early merging event rather than that of an ultra-luminous infrared galaxy. And the size of the star clusters formed as a consequence of the interaction now agree with those of clusters created in other mergers instead of being 1.5 times as large.

So the question is, why was the distance of such a well-studied galaxy system so far off in the first place? “The distance was never directly measured before,” reveals Saviane, “People took the recession velocity of the galaxies and divided by the Hubble constant, which gives a first order guess of the distance.” However, it seems that the problem was further enhanced by the fact that different groups had used different values for the Hubble constant as well as for the recession velocity, which has to be corrected to the reference frame of the cosmic microwave background radiation: different groups had made different corrections. “This is why a distance based on a standard candle was giving a discrepant result,” says Saviane.